5.1 Meiosis, 5.2 Meiosis and Genetic Diversity

Genetics

the study of heredity and heredity variation

Heredity

the transmission of traits from one generation to the next

traits are passed from parent to offspring through genes (segments of DNA that code for heredity, offspring acquire genes from parents by inheriting chromosomes)

Asexual Reproduction

single individual

no fusion of gametes

clones( offspring are exact copies of the parent)

mutations are the only source of variation

can produce asexually through mitosis

Sexual Reproduction

two parents (male/female)

offspring are unique combinations of genes from parents

genetically varied from parents and siblings

meiosis

Homologous chromosomes

a pair of chromosomes (same size, length, centromere position) that carry the same genetic information

one homologous chromosome is inherited from mom and one is inherited from dad

Karyotype

a display of chromosome pairs ordered by size and length

used to see irregularities like down syndrome, turner syndrome, and klinefelter syndrome

Somatic (body) cells

diploid or 2n

two complete sets of each chromosome

humans: 2n=46

Gametic (sex) cells

haploid or n

one set of each chromosome

humans(sperm and egg) n=23

eukaryotes have DNA that is packaged in chromosomes

there are 2 types of chromosomes

autosomes- chromosomes that do not determine sex (humans have 22 pairs)

sex chromosome is 23rd pair

sex chromosomes- x and y

eggs- X(humans 22+X)

sperm- X or Y (humans 22+X or 22+Y)

all sexually reproducing organisms have both a diploid and haploid number

Life cycle

sequence of stages in the reproductive history of an organism from conception to its own reproduction

fertilization and meiosis alternative in sexual life cycles

fertilization is when a sperm cell (haploid) fuses with an egg (haploid) to form a zygote (diploid)

Meiosis

a process that creates haploid gametes cells in sexually reproducing diploid organisms

results in daughter cells with half the number of chromosomes as the parent cell

ex. humans have a diploid, 2n=46→meiosis produces sperm and eggs that are haploid n=23

involves two rounds of division

meiosis I and meiosis II

Mitosis vs Meiosis

Mitosis- occurs in somatic cells, 1 division, results in 2 diploid daughter cells, daughter cells are genetically identical

Meiosis- forms gametes(sperm/egg), 2 divisions, results in 4 haploid daughter cells, each daughter cells is genetically unique

Key events in meiosis

3 key events in meiosis that are unique

1. Prophase I- synapsis and crossing over

2. Metaphase I- tetrads (homologous pairs) line up at the metaphase plate

3. Anaphase I- homologous pairs separate

Meiosis I

Interphase→Prophase I→Metaphase I→Anaphase I→Telophase I and Cytokinesis

Interphase

cell goes through G1, S(DNA is copied), and G2

Prophase I

homologous chromosomes condense and pair up in a process known as synapsis

the homologous pairs (tetrads) are held together by a protein framework called the synaptonemal complex

meiotic spindle begins to form

centrosomes move to opposite poles of the cell

nuclear envelope breaks down

crossing over (recombination): DNA is exchange between the non-sister chromatids

-physical X-shaped connections where sites of recombination/crossing over occurred are called chiasmata

-produces recombinant chromatids, every chromatid that is produced by a unique combinations of DNA

Metaphase I

independent orientation- meiotic spindle fibers align tetrads at the metaphase plate

chromosomes can independently orient themselves in 8 million different ways here

Anaphase I

pairs of homologous chromosomes separate as meiotic spindle fibers pull them towards poles

sister chromatids are still attached

Telophase I and Cytokinesis

meiotic spindle breaks down

new nuclear envelope develops

cleavage furrow or cell plate forms

cytokinesis occurs

there is now a haploid set of chromosomes in each daughter cell

Meiosis II

prophase II→metaphase II→anaphase II→ telophase II and cytokinesis

Prophase II

no crossing over

meiotic spindle forms

sister chromatids connected at the centromere attach to meiotic spindle

Metaphase II

chromosomes line up at the metaphase plate

because of crossing over in meiosis I, the chromatids are unique

Anaphase II

proteins at the centromeres break down

sister chromatids separate and move towards opposite poles

Telophase II and Cytokinesis

meiotic spindle breaks down

new nuclear envelope develops

cleavage furrow or cell plate forms

chromatids begin to decondense

cytokinesis occurs

4 genetically unique haploid cells

How does meiosis lead to genetic variation?

crossing over, independent assortment, random fertilization

Crossing over

happens in prophase I

produces recombinant chromosomes: they exchange genetic material

helps to create genetic variation

Independent assortment

chromosomes are randomly oriented along the metaphase plate during Metaphase I

each can orient with either the maternal or paternal chromosomes closer to a given pole

creates genetic variation

Random Fertilization

any sperm can fertilize any egg

creates genetic variation

Nondisjunction

when homologous chromosomes in meiosis I or sister chromatids in meiosis II separate incorrectly and the resulting gametes are not haploid

the gametes may carry an extra chromosome(n+1) r lack one altogether(n-1)

ex. down syndrome, 3 copies of chromosomes 21